The present invention relates generally to a system for transfer of charged toner particles in an electrostatographic printing apparatus, and more particularly concerns a method and apparatus for enabling transfer of charged developing material to an intermediate transfer member by applying an oscillatory bias voltage to the charged developing material.
Generally, the process of electrostatographic image reproduction is executed by exposing a light image of an original document onto a substantially uniformly charged photoreceptive member. Exposing the charged photoreceptive member to a light image discharges a photoconductive surface thereon in areas corresponding to non-image areas in the original document while maintaining the charge in image areas, thereby creating an electrostatic latent image of the original document on the photoreceptive member. Charged developing material is subsequently deposited onto the photoreceptive member such that the developing material is attracted to the charged image areas on the photoconductive surface is thereof to develop the electrostatic latent image into a visible image. The developing material is then transferred from the photoreceptive member, either directly or after an intermediate transfer step, to a copy sheet or other support substrate, creating an image which may be permanently affixed to the copy sheet to provide a reproduction of the original document. In a final step, the photoconductive surface of the photoreceptive member is cleaned to remove any residual developing material thereon in preparation for successive imaging cycles.
Analogous processes also exist in other electrostatographic printing applications such as, for example, ionographic printing and reproduction, where charge is deposited in an image pattern on a charge retentive surface in response to electronically generated or stored images, as described in U.S. Pat. Nos. 3,564,556; 4,240,084; and 4,619,515 among others.
The process of transferring developing material from an image support surface to a second supporting surface is typically realized at a transfer station. In a conventional transfer station, transfer is achieved by applying electrostatic force fields in a transfer region sufficient to overcome forces which hold the toner particles to the photoconductive surface on the photoreceptive member. These electrostatic force fields operate to attract and transfer the toner particles over onto the second supporting surface which may be an intermediate transfer belt or an output copy sheet. An intermediate transfer belt is desirable for use in tandem color or one pass paper duplex (OPPD) applications where successive toner powder images are transferred onto a single copy sheet. For example, U.S. Pat. No. 3,957,367 issued to Goel, the disclosure of which is incorporated herein by reference, teaches a color electrostatographic printing machine wherein successive single-color powder images are transferred to an intermediary, in superimposed registration with one another. The resultant multi-layered powder image is subsequently transferred to a sheet of support material to form a color copy of an original document. Color and OPPD systems may also utilize multiple photoconductive drums in lieu of a single photoconductive drum.
Intermediate transfer elements employed in imaging systems of the type in which a developed image is first transferred from the imaging member to an intermediate member and then transferred from the intermediate to an outer copy substrate should exhibit efficient transfer characteristics both for transfer of the developer material from the imaging member to the intermediate as well as for transfer of the developer material from the intermediate to the output copy substrate. Efficiency of transfer is determined by the percentage of the developer material comprising the developed image is transferred with respect to the residual developer remaining on the surface from which the image was transferred. Highly efficient transfer is particularly important when the imaging process entails the creation of full color images by sequentially generating and developing successive images in each primary color and superimposing the developed primary color images onto each other during transfer to the substrate. In particular, undesirable shifting and variation in final colors produced can occur when the primary color images are not efficiently transferred to the substrate.
Conventional transfer of toner images between support surfaces in electrostatographic applications is often accomplished via electrostatic induction or by applying a potential difference between the substrate of a biased member contacting the second supporting member and the image bearing surface originally supporting the toner image layer. Such transfer process focuses on applying and maintaining high intensity electric fields in the transfer region in order to overcome the adhesive forces acting on the toner particles. Careful control of these electric fields is required to induce the physical detachment and transfer-over of the charged particulate toner materials from one surface to a second supporting surface without scattering or smearing of the developer material. The electric fields across the transfer region must be controlled so that the fields are high enough to effect efficient toner transfer while being low enough so as not to cause arcing, excessive corona generation, or excessive toner transfer in the regions prior to intimate contact of the second supporting surface and the toner image. Imprecise and inadvertent manipulation of these electric fields can create copy or print defects by inhibiting toner transfer or by inducing uncontrolled toner transfer, causing scattering or smearing of the toner particles.
Various problems associated with conventional image transfer are well known. Variations in conditions, such as second supporting surface resistivity, contaminants, and changes in the toner charge or in the adhesive properties of the toner materials, can all effect necessary transfer parameters. Further, material resistivity and toner properties can change greatly with humidity and other ambient environmental parameters. In the pre-nip gap or so called pre-nip region, immediately in advance of contact between the substrate surface and the developed image, excessively high transfer fields can result in premature transfer across an air gap, leading to decreased resolution or blurred images. High transfer fields in the pre-nip gap can also cause ionization which may lead to strobing or other image defects, loss of transfer efficiency, and a lower latitude of system operating parameters. Conversely, in the post-transfer nip gap or so called post-nip region, at the photoconductor/second supporting surface separation area, insufficient transfer fields are considered to cause image dropout and may generate hollow characters. Also, improper ionization in the post-nip region may cause image stability defects or can create copy sheet detacking problems.
Induced variations in field strength across the transfer region can be considered contrary to a conventional premise that the transfer fields should be as large as possible in the region directly adjacent to the transfer nip, where the second supporting surface contacts the developed image, so that high transfer efficiency and stable transfer are expected to be achieved.
However, in accordance with the present invention, an apparatus for transferring charged image developer material from an image support surface to a substrate is provided, wherein a substrate is positioned to have at least a portion thereof adjacent the image support surface to define a transfer region including a pre-nip region, a transfer nip, and a post-nip region and a transfer station, located adjacent the transfer region, is provided for establishing an oscillatory voltage potential between the image support surface and the substrate so as to establish an oscillatory electric field in the transfer nip. The induced oscillatory electric field is of the appropriate field strength and exhibits an oscillatory (bidirectional) component having alternating polarity and a constant (unidirectional) component having a single polarity that is appropriate for the ultimate toner transfer direction, so as to cause repeated transfer and back transfer of the toner within the transfer nip in a fluidized motion to and from the substrate. The oscillatory mode of the applied oscillatory electric field diminishes to a selected level, such that the constant component is sufficient to effect high transfer efficiency in the ultimate toner transfer.